Polymeric materials are used in medical endoprostheses, e.g., orthopaedic implants (e.g., hip replacement prostheses). For example, ultrahigh molecular weight polyethylene (UHMWPE) is used to form components of artificial joints. Desirable characteristics for the polymeric materials used in medical endoprostheses include biocompatibility, a low coefficient of friction, a relatively high surface hardness, and resistance to wear and creep. It is also desirable for such endoprostheses to be readily sterilizable, e.g., by using high-energy radiation, or by utilizing a gaseous sterilant such as ethylene oxide, prior to implantation in a body, e.g., a human body.
High-energy radiation, e.g., in the form of gamma, x-ray, or electron beam radiation, is often a preferable method of sterilization for some endoprostheses because, in addition to sterilizing the endoprostheses, often the high energy radiation crosslinks the polymeric materials, thereby improving the wear resistance of the polymeric materials. However, while treatment of some endoprostheses with high-energy radiation can be beneficial, high-energy radiation can also have deleterious effects on some polymeric components. For example, treatment of polymeric components with high-energy radiation can result in the generation of long-lived, reactive species within the polymeric matrix, e.g., free radicals, radical cations, or reactive multiple bonds, that over time can react with oxygen, e.g., of the atmosphere or dissolved in biological fluids, to produce oxidative degradation in the polymeric materials.
Such degradation can reduce the wear resistance of the polymeric material. Therefore, it is often advantageous to reduce the number of such reactive species. Radiation sterilization of polymeric materials, crosslinking, and entrapment of long-lived, reactive species, and their relationship to wear are discussed in Kurtz et al., Biomaterials, 20, 1659-1688 (1999); Tretinnikov et al., Polymer, 39(4), 6115-6120 (1998); Maxwell et al., Polymer, 37(15), 3293-3301(1996); Kurtz et al., Biomaterials, 27, 24-34 (2006); Wang et al., Tribology International, 31(1-3), 17-33 (1998); Oral et al., Biomaterials, 26, 6657-6663 (2005); Oral et al., Biomaterials, 25, 5515-5522 (2004); Muratoglu et al., Biomaterials, 20, 1463-1470 (1999); Hamilton et al., European Patent Application No. 1072276A1; Li et al., U.S. Pat. No. 5,037,928, NcNulty et al., U.S. Pat. No. 6,245,276; and Muratoglu et al., PCT Publication No. WO 2005/074619.